Polythiophenes have been studied extensively due to their interesting electrical and/or optical properties. Polythiophenes become electrically conducting upon chemical or electrochemical oxidation or reduction. Their ultimately achievable electrical conductivity is determined by their chemical composition, the stereoregularity of the polymerization of the thiophene monomers in the polythiophene chain and by their π-conjugation lengths. Such stereoregularity problems do not arise when unsubstituted thiophenes or thiophenes substituted in the 3- and 4-positions with identical groups are polymerized.
EP-A 339 340 discloses a polythiophene containing structural units of the formula:
in which A denotes an optionally substituted C1-C4-alkylene radical and its preparation by oxidative polymerization of the corresponding thiophene.
EP-A 440 957 discloses dispersions of polythiophenes, constructed from structural units of formula (I):
in which R1 and R2 independently of one another represent hydrogen or a C1-C4 alkyl group or together form an optionally substituted C1-C4-alkylene residue, in the presence of a polyanion compound and specifically discloses in Examples the polymerization of 3,4-ethylenedioxythiophene [EDOT] in the presence of poly(styrene sulphonic acid) at a weight ratio of 1:1.29 (Example 2), 1:2.2 (Examples 7 and 8), 1:4 (Examples 1, 3, 4 and 10), 1:6 (Example 5) and 1:8.33 (Example 6) in water.
In 2001, Elschner et al. at ASIA DISPLAY/IDW′0 held at Nagayo in Japan in October 2001 in paper OEL3-3 disclosed high resistivity PEDOT/PSS for reduced crosstalk in passive matrix OEL's and presented characteristic data of standard BAYTRON™ P and BAYTRON™ P types AI4083 and CH8000, reproduced below:
PSS:PEDOTBAYTRON PResistivityCentre of particlecomposition byType[Ω-cm]size distribution [nm]weightstandard11102.5:1VP AI4083500-100055 6:1VP CH800070,000-200,00020 20:1Elschner et al. further report that the conductivity decreased by several orders of magnitude relative to standard BAYTRON™ P just be shifting the centre of the particle size distribution from about 100 nm to 20 nm, while noting that the sizes of gel particles are taken in their aqueous swollen state and are of smaller size after dehydration. BAYER's current commercial literature gives the following characteristics for these three BAYTRON™ P grades:
BAYTRON PResistivityparticle size distribution (swollen)Type[Ω-cm]d50 [nm]d90 [nm]d95 [nm]VP AI40831000<200VP CH8000100,000<35<60<200
Aqueous dispersions of PEDOT/PSS are commercially available from BAYER as BAYTRON™ P. The molar ratio in standard BAYTRON™ P analyzed by 13C NMR analysis, as described in 2002 by P. Adriaensens et al. in Polymer, volume 43, pages 7003 to 7006, was found to be 2.7 (=weight ratio of 3.50) and that in standard aqueous dispersions of PEDOT/PSS commercialized by AGFA-GEVAERT N.V. was found to be 2.1 (=weight ratio of 2.72).
TABLE 1InitialInitialAverageInitialaromaticaliphaticinitialethercarboncarboncarboncarbonatomatomatomatomintensityintensityintensityintensityPEDOT typeof PSSof PSSof PSSof PEDOTPSS/PEDOTBAYTRON ™ P Sample 1445454449.51642.7BAYTRON ™ P Sample 2246260253853.0AGFA-GEVAERT4464934702262.1PSS/PEDOTdispersion
In 1999, M. Lefebvre et al. in Chem. Materials, volume 11, pages 262-268, reported that the electrical conductivity of PEDOT/PSS prepared from aqueous and acetonitrile (AN) solutions of EDOT and NaPPS with PSS to PEDOT ratios ranging from 0.24 to 3.33 increased with decreasing PSS:PEDOT ratio as would be intuitively expected by one skilled in the art, due to the higher concentration of the intrinsically conductive component PEDOT.
Solvent usedPSS:PEDOT ratio obtainedInitial conductivity [S cm−1]AN/water0.481.3AN/water0.671.0AN/water0.711.5AN/water0.910.3AN/water1.02.5AN/water3.336 × 10−3water0.249.9water2.00.3water2.50.4
EP-A-686 662 discloses mixtures of A) neutral polythiophenes with the repeating structural unit of formula (I),
in which R1 and R2 independently of one another represent hydrogen or a C1-C4 alkyl group or together represent an optionally substituted C1-C4 alkylene residue, preferably an optionally with alkyl group substituted methylene, an optionally with C1-C12-alkyl or phenyl group substituted 1,2-ethylene residue or a 1,2-cyclohexene residue, and B) a di- or polyhydroxy- and/or carboxy groups or amide or lactam group containing organic compound; and conductive coatings therefrom which are tempered to increase their resistance preferably to <300 ohm/square. Furthermore, the examples in EP-A 686 662 disclose the polymerization of EDOT in the presence of poly(styrene sulphonic acid) at a weight ratio of 1:3.57 in water.
WO 03/001299A discloses a material for making an electroconductive pattern, said material comprising a support and a light-exposure differentiable element, characterized in that said light-exposure differentiable element comprises an outermost layer containing a polyanion and a polymer or copolymer of a substituted or unsubstituted thiophene, and optionally a second layer contiguous with said outermost layer; and wherein said outermost layer and/or said optional second layer contains a light-sensitive component capable upon exposure of changing the removability of the exposed parts of said outermost layer relative to the unexposed parts of said outermost layer. WO 03/001299A further discloses that conductivity enhancement refers to a process in which the conductivity is enhanced e.g. by contact with high boiling point liquids such as di- or polyhydroxy- and/or carboxy groups or amide or lactam group containing organic compound optionally followed by heating at elevated temperature, preferably between 100 and 250° C., during preferably 1 to 90 seconds, results in conductivity increase. Alternatively in the case of aprotic compounds with a dielectric constant≧15, e.g. N-methyl-pyrrolidinone, temperatures below 100° C. can be used. Such conductivity enhancement is observed with polythiophenes and can take place during the preparation of the outermost layer or subsequently. Particularly preferred liquids for such treatment are N-methyl-pyrrolidinone and diethylene glycol such as disclosed in EP-A 686 662 and EP-A 1 003 179. The PEDOT/PSS dispersion used in all the EXAMPLES of WO 03/001299A had a PEDOT:PSS weight ratio of 1:2.4 and had a narrow particle size distribution determined by CPS disc centrifuge measurements with a maximum at 25 nm and an average particle size of 30-50 nm.
EP-A 1 003 179 discloses a method for producing a polymeric conductive layer on an object comprising the steps of: providing an aqueous composition containing a polythiophene, a polyanion compound and an aprotic compound with a dielectric constant, ∈, ≧15; applying said composition to said object forming a layer; and drying said layer to form a conductive polymeric layer on said object, characterized in that said object and said layer are kept at a temperature below 100° C. and said conductive polymeric layer has a resistivity of at most 2 kΩ/square. Furthermore, the examples in EP 1 003 179 disclose the polymerization of EDOT in the presence of poly(styrene sulphonic acid) at a weight ratio of 1:2.46 in water.
Furthermore, in 2002 Kim et al. in Proceedings of SPIE, volume 4464, pages 85-92, reported a dramatic increase in conductivity of PEDOT/PSS without losing optical transparency by addition of a small amount of a polyalcohol, e.g. glycerol, in which BAYTRON™ P with a mean particle size in the range of 25 nm-75 nm was used as the starting material and Pettersson et al. in Organic Electronics, volume 3, pages 143-148, reported an increase in conductivity of PEDOT/PSS without losing transparency by addition of sorbitol in which the starting material was also BAYTRON™ P i.e. a weight ratio of PEDOT:PSS of 1:2.5.
A general drawback of conductive polymers which have been prepared and studied up to now, is that their conductivities are still too low for certain applications, their visible light transmittances are insufficiently high and/or they are not processable.